1. Field of the Invention
This invention relates to the removal of alumina from aqueous alkali metal chromate solutions containing the same, and more specifically, to the removal of alumina from said solutions by reaction with soluble silica.
2. Description of the Prior Art
Hexavalent chromium compounds such as alkali metal chromates (e.g., sodium chromate), alkali metal bichromates (e.g., sodium bichromate) and other chrominum compounds, useful in such diverse industries as the plating of metals, tanning of leather and the manufacture of pigments, are typically produced from chromite ore such as FeOCr.sub.2 O.sub.3. The ore is generally subjected to a high temperature alkaline oxidation (e.g., temperatures in the order of about 2,000.degree. to 2,500.degree. F.), usually in the presence of an alkali metal carbonate (e.g., soda ash and/or potassium carbonate) or alkali metal hydroxide (e.g., sodium hydroxide) with the consequent formation of alkali metal chromate. Other alkaline materials such as lime (or other CaO-containing material) and MgO may also be added.
The oxidation step is then followed by water leaching of the oxidized ore to produce a "yellow liquor" comprising an aqueous solution of alkali metal chromate, contaminated with water-soluble impurities such as alumina which are originally present in the ore. Subsequent treatment of the yellow liquor to obtain the desired hexavalent chromium compounds varies widely. The yellow liquor has been treated with an acidic material such as sulfuric acid to oxidize the alkali metal chromate to bichromate, thereby forming a solution of alkali metal bichromate and by-product alkali metal sulfate, containing the water-soluble impurities such as alumina carried to the yellow liquor during leaching. In other processes, the yellow liquor has been treated by contacting the liquor with carbon dioxide as the acidic material, in which case alkali metal bicarbonate rather than alkali metal sulfate is formed as by-product in the resultant bichromate solution. (See, e.g., U.S. Pat. No. 1,760,788). Likewise, by-product alkali metal bisulfate (see, e.g., U.S. Pat. No. 3,715,425) and alkali metal bichromate (see, e.g., Encyclopedia of Chemical Technology, Vol. 3, pg. 50; Kirk & Othmer, eds.; 1954) have been used as the acidic material in place of the sulfuric acid in the treatment of the yellow liquor for production of the bichromates.
In such processes, recovery of the bichromate is generally effected by concentrating the aqueous solution containing the bichromate to precipitate the by-product alkali metal sulfate or bicarbonate (depending on which acidic material is employed to treat the yellow liquor), removing the precipitate from the concentrated solution, followed by crystallizing alkali metal bichromate from the purified concentrated liquor. However, the concentration step generally also results in removal of a portion of the alumina from solution, which forms a suspension of alumina. Since the presence of by-product alumina in the crystallization step results in contamination of the bichromate crystals, removal of the alumina precipitate from the concentrated solution prior to crystallization is necessary. The presence in the concentrated solution of even very small quantities of suspended alumina greatly impairs the efficiency of separation of this precipitate from the aqueous solution.
To avoid alumina separation problems the prior art has sought to convert the alumina in the chromite ore to an insoluble form during the roasting step by adding lime or other materials to the roast mix. Such high lime roasts generally form a low-level alumina yellow liquor upon subsequent leaching, i.e. containing from about 0.35 to 1.0 weight percent CTB Al.sub.2 O.sub.3 (as this term is defined below). However, this process has never been entirely successful due to the difficulty in obtaining the precise conversion desired. Deficiencies of lime result in a yellow liquor containing excessive amounts of solubilized alumina, while excess of lime causes the physical consistency of the roast to be changed during leaching from a granular, porous material to a slimy, non-draining form, which seriously impairs the leaching operation. To avoid the formation of the non-draining form of roast, careful and strict control of the amount of lime relative to the quantity of alumina in the ore is necessitated, thereby leading to increased expenses in processing the ore.
To allow use of lower lime quantities, other processes have employed higher amounts of alkali metal carbonates and/or hydroxides in the chromite ore mix roasting step to open up the ore. However, the increased alkali metal carbonate and/or hydroxide increases the amount of alumina solubilized in the yellow liquor. Moreover, any unreacted sodium carbonate is converted during roasting to anhydrous sodium hydroxide (Na.sub.2 O), which then forms sodium hydroxide in the subsequent leaching step, leading to a substantial increase in the alkalinity of the leach liquor. Raw material costs in the subsequent acidification step (to convert chromate values to bichromate) are correspondingly increased.
Other processes dealing with the removal of alumina include:
(1) British Pat. No. 1,057,678 (1967) employs a no- or low-lime roast and leaches the roast with acid solutions to a pH of 4 to 9.
(2) U.S. Pat. No. 1,760,788 (1930 to Specketer et al.) admixes bauxite or other alumina-rich material and soda ash with the chromite ore; roasts to form sodium aluminate; extracts the roast with water to form a leach liquor containing dissolved sodium aluminate and sodium chromate and smaller amounts of sodium silicate, caustic soda and soda ash; and heats the leach liquor to at least 100.degree. C. to react the dissolved sodium silicate with a certain quantity of alumina whereby the silicic acid is almost completely separated in the form of sodium-aluminum silicate. Alumina is stirred out after removal of the sodium-aluminum silicate and separation is completed by introducing CO.sub.2. Alternatively, the leach liquor may be first contacted with CO.sub.2 to separate alumina.
(3) U.S. Pat. No. 1,948,143 (1934 to Tarr) roasts the ore together with an alkaline earth refractory material (e.g., lime), an alkali base (e.g., soda ash) and leached residue from the subsequent step containing an alkaline earth metal chromate, using a two-stage roast.
(4) U.S. Pat. No. 3,899,568 (1975 to Frick et al.) acidifies alkaline chromate leach liquors in two stages to precipitate alumina as a more easily separable alumina trihydrate.
The two-stage roast of Tarr and the two-stage acidification of Frick et al. require careful control to achieve the desired separation of alumina, and the processes of British Pat. No. 1,057,678 and Specketer et al. do not avoid the problem of separating the finely precipitated alumina. Indeed, Specketer et al. add to the difficulty of alumina removal by employing an alumina-rich diluent (bauxite) in the roasting stage.